1. Field of the Invention
This invention relates to a centralized, multistation, powerline carrier control system having a powerline carrier transmitter. More particularly, it relates to a carrier transmitter which employs a pulse inverter and generates a keyed audio frequency output voltage in forming transmitting pulses, the output being fed, via a coupling unit, into an AC power distribution network.
2. Description of the Prior Art
Ways are known by which information may be transmitted to power users by means of a centralized powerline control system in which the single or multiple phase electric distribution network serves as the transmission path. Such information may serve, for instance, for initiating switching actions in distribution networks, for switching users' watt/hour meters to different rates (for instance, high and low rates), or for notifying a person or persons (for instance, the fire department).
A centralized, multistation, powerline carrier control system consists essentially of a power line carrier transmitter, a coupling unit network for feeding the signal into the distribution network, and of one or more powerline carrier receivers which are connected to the distribution network and can be used for control purposes at the user's location. The carrier transmitter generates an audio frequency (AC) output voltage coded in accordance with the information to be transmitted, which is superimposed by the coupling unit on the AC line voltage in the electric power network. Selective powerline carrier receivers connected to the network decode the transmitted signals and perform the tasks for which they were installed. The frequency of the voltage being distributed is usually 50 or 60 Hz, and the audio frequency chosen is generally between 110 and 400 Hz.
A powerline carrier control system of this type is described in "Siemens-Zeitschrift" 48 (1974), pages 69 to 75, which shows a coupling network for either a series feed or parallel feed (loc. cit., FIG. 2). The parallel feed unit can comprise and L-C combination and an isolation transformer; in the three-phase case, a series circuit having a series choke and a series capacitor is arranged in each lead on the network side of the isolation transformer, and a compensating capacitor is placed between each of the three leads on the transmitter side (loc. cit., FIG. 3). Or, the series feed unit can be an L-C unit and a coupling (feed) transformer; in the three-phase case, three delta-connected, shunt capacitors are connected, on the one hand, to the lines between the carrier transmitter and the coupling transformers, the coupling transformers being connected on the other hand, to one of each of three chokes, the other ends of which are connected to an appropriate junction point of three more delta-connected capacitors (loc. cit., see FIG. 4).
German Offenlegungsschrift No. 24 56 344 (U.S. Pat. No. 4,021,797) shows a powerline carrier control system which uses, as a powerline carrier control transmitter, a static frequency changer having a linefed diode rectifier, an intermediate voltage circuit with a capacitor, and a three-phase, pulsed inverter connected thereto, the pulsed inverter being constructed from power thyristors.
In powerline carrier control systems of the type just described, an undesired lengthening of a code element or transmitter pulse occurs when the transmission of the signal voltage stops because the entire L-C combination (in the case of parallel feed) or the L-C unit (in the case of series feed) continues to oscillate, or "ring", for several periods. This unwanted lengthening of each transmitted signal results in falsification (distortion) of the transmitted information, and can lead to error in operation (malfunction) of the powerline carrier control receivers. Such lengthening occurs in both series and parallel feed units. However, it has been found that the lengthening of the transmitter pulses is especially large in parallel feed units and is particularly troublesome there.
It is an object of the invention to substantially, if not completely, eliminate such distortion of the transmitter pulse in the transmission of information on a powerline carrier control system, particularly one having a parallel feed unit, by measures taken on the transmitter side.